The present invention concerns a technique for initializing an integrated switched mode power supply.
A flyback power supply with an offline regulator circuit may typically use a high voltage switching transistor within an integrated circuit to control current through the primary winding of a transformer, thereby regulating a second voltage across the secondary winding of the transformer. At power-up of the power supply, it is necessary to initialize the regulator circuit and the high voltage switching transistor thereon. Typically this is done with start-up circuitry within the regulator circuit. The start-up circuitry may typically include a high voltage power up transistor and require a separate high voltage pin which is connected to an off-circuit high voltage D.C. source.
The addition of a high voltage power up transistor and associated separate high voltage pin has severe disadvantages. For example, the integrated circuit has to be designed so that the high voltage pins are spaced at sufficient distance to prevent current creepage between them. Further, it is desirable to design the high voltage power up transistor so that it is small and thus allows only a few milliamps of current. However, small high voltage transistors are difficult to protect against electric static discharge.